P435/NLT–specific CTLs target primary tumor cells and tumor progenitors. We performed clonogenic assays using peripheral and bone marrow MNCs from patients with CML to test antitumor activity by PRAME-CTLs against tumor progenitors. (A) CFUs generated from MNCs of PRAME⁺HLA-A*02⁺ and PRAME⁺HLA-A*02⁻ CML patients with active disease and HLA-A*02⁺CD34⁺ cells from healthy donors incubated with P435/NLT–specific CTLs, P4-specific CTLs, or control CTLs (targeting the MART1-derived ELA epitope or the pp65-derived NLV peptide). P435/NLT–specific CTLs had a specific cytotoxic activity against PRAME⁺HLA-A*02⁺ leukemic precursors, because the number of CFUs was significantly lower when MNCs were coincubated with P435/NLT–specific CTLs compared with control CTLs (P = .01). (B) Expression of PRAME and the oncogenic fusion BCR-ABL (p210) transcript using Q-RT-PCR in colonies collected from 2 independent experiments. P435/NLT–specific CTLs almost completely eliminated the growth of PRAME⁺ CFUs and significantly reduced the growth of BCR/ABL⁺ CFUs.